The present invention relates to an ignition device for an internal combustion engine and, particularly, to a device for limiting a primary current of an ignition coil thereof.
FIG. 3 shows an example of a conventional ignition device of this type, in which a reference numeral 1 depicts a power source, 2 an ignition coil, 3 an ignition device and 4 and 5 are a resister and a transistor, respectively, constituting a circuit for producing a drive signal for the ignition device.
The ignition device 3 includes an output terminal 31 connected to the ignition coil 2, a grounding terminal 32 and an input terminal 33 connected to the circuit. When the transistor 5 of the circuit is on/off operated, a signal is supplied to the input terminal 33 of the ignition device 3 such that, upon a turn-off of the transistor 5, a current flows from the power source 1 through the resister 4 and an internal resistance 303 of the ignition device to a base of a Darlington power transistor 301 to turn the latter on to thereby supply a primary current to the ignition coil 2.
A primary current detecting resister 302 is provided between an emitter of the power transistor 301 and a grounding point so that a voltage across the resister 302 increases with an increase of the primary current.
A transistor 307 has a base connected through a resister 304 to the emitter of the power transistor 301, an emitter grounded and a collector connected to the base of the power transistor 301. Between the base and the emitter of the transistor 307, a circuit constituted with a resister 305 and a transistor 306 is connected. When a voltage across the primary current detecting resister 302 exceeds a turn-on voltage of the transistor 307, a current flows through the resister 304 to the base of the transistor 307 and the resister 305. The collector of the transistor 307 absorbs a portion of the base current of the power transistor 301 correspondingly to a degree of conduction of the transistor 307.
The primary current of the ignition coil is limited to a constant value when a balance condition determined by the base current of the power transistor 301, the voltage across the primary current detecting resister 302, the base current of the transistor 307 and a current amplification factor of the transistor circuit composed of the power transistor 301 and the transistor 307 is satisfied.
The collector and the base of the transistor 306 are short-circuited so that it functions as a diode. That is, a temperature dependency of the base-emitter voltage of the transistor 307 is compensated for by a temperature dependency of base-emitter voltage of the transistor 306 to thereby solving a temperature dependency problem of current limitation.
With such scheme as mentioned above, the primary current of the ignition coil is limited to a constant value which is just enough for ignition, allowing a use of a relatively small power transistor.
In the conventional device mentioned above, however, the constant primary current means that a current amount to be absorbed by the transistor 307 is constant, while the base current of the transistor 301 varies with a variation of the source voltage. That is, it is impossible to obtain a constant current limitation value when the source voltage varies. For example, when the source voltage increases, the base current of the power transistor 301 increases correspondingly. In order to absorb a current increment by means of the transistor 307, it is necessary to increase the voltage across the primary current detecting resister, i.e., to increase the primary current, requiring a large power transistor. On the contrary, when the source voltage decreases, the current limit value is lowered, causing an output of a secondary coil of the ignition coil to be lowered or a heat generation problem to occur.
It is usual that the transistor 306 provided for compensation of temperature denpendency of the current limit value is not enough to cancel out a temperature dependent variation of a base-emitter voltage of the transistor 307 and that, due to the fact that the primary current detecting resister 302 is of a metal having resistance varying with temperature, the current limit value is large at low temperature and small at high temperature.